This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-189938, filed Jun. 23, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an antenna capable of tracking a number of communication satellites simultaneously and a waveguide available to transmission of transmit and receive signals associated with the antenna.
2. Description of the Related Art
At present about 200 communication satellites travel around the earth in low earth orbits. Thus, it is possible to communicate with at lest several satellites at any point on the earth. Satellite-based communication systems include the IRIDIUM system and the SKY BRIDGE system.
As antennas for communication satellites, parabolic antennas and phased-array antennas have heretofore been used widely.
An example of a parabolic antenna system is illustrated in FIGS. 1 and 2. The parabolic antenna system of FIG. 1 includes a post 101 set upright on the ground or the floor of a building, a shaft of rotation 102 attached to the upper portion of the post 101 in parallel so that it can revolve around the post, a gear 103g mounted to the rotation shaft 102, and a gear 103 which engages with the gear 102g and is rotated by a motor not shown.
The upper portion of an electromagnetic-wave focusing unit (hereinafter referred to as the reflector unit) 120 is attached to the top of the shaft 102 through a bracket 111 so that it can rotate in the up-and-down direction. The lower portion of the reflector unit 120 is attached to the end of a rod 112a in a cylinder unit 112 mounted to the lower portion of the shaft 102. A feed 130 is placed at the point at which electromagnetic waves are focused.
The parabolic antenna 100 thus constructed allows the azimuth of the reflector unit 120 to be controlled by driving the motor to thereby cause the shaft 102 to revolve around the post 101 through the gears 103 and 102g. On the other hand, the angle of elevation of the reflector unit 120 can be controlled by driving the cylinder unit 112. In this manner, the parabolic antenna can orient its reflector unit 120 to a communication satellite to transmit or receive electromagnetic waves to or from the satellite under good conditions.
However, with the conventional parabolic antenna system, one feed 130 is associated with one reflector unit 120. If there are two satellites to be tracked, therefore, the same number of parabolic antenna systems are required.
Two parabolic antenna systems must be placed so that they do not interfere with each other. For example, when the reflector unit 120 has a circular shape and measures 45 cm in diameter, two reflector units must be placed on the horizontal plane at a distance of about 3 m apart from each other as shown in FIG. 2 in order to prevent one reflector unit from projecting its shadow on the other.
However, such an antenna system as shown in FIG. 2 requires a large space for installation and is therefore not suited for household use.
As described above, the conventional antenna apparatus capable of tracking two communication satellites simultaneously requires large space for installation. An antenna apparatus which is capable of tracking two communication satellites which is compact and requires less installation space is therefore in increasing demand.
With such an antenna apparatus, to make it compact, it is required to bend a waveguide used to couple a transmit-receive module and a primary radiator (feed) together. However, since two perpendicularly polarized waves of different frequencies are used for transmit and receive signals, it is required to prevent electrical characteristics from degrading in waveguide bends.
It is therefore an object of the present invention to provide an antenna apparatus which is capable of tracking two satellites simultaneously which is so compact that it can be installed in relatively small space.
It is another object of the present invention to provide a waveguide which, in transmitting two perpendicularly polarized waves of different frequencies, prevents electrical characteristics from degrading in its bends.
To attain the first object, an antenna apparatus of the present invention comprises: a fixed base having a datum plane and fixed in an installation place; a rotating base placed on the fixed base and adapted to be rotatable about a Z axis perpendicular to the datum plane; a support rail in the shape of substantially a semicircular arc, the rail being placed over the rotating base and adapted to be rotatable about a Y axis perpendicular to the Z axis with its central point on the Z axis and the Y axis passing through the central point of the support rail; first and second rotating shafts provided between an end of the support rail and the central point and between the other end of the support rail and the central point, respectively, to form an X axis perpendicular to the Y axis and adapted to be rotatable about the X axis independently of each other; first and second antennas fixed to the first and second rotating shafts, respectively; a Z-axis rotating mechanism for allowing the fixed base to rotate about the Z axis; a Y-axis rotating mechanism for allowing the support rail to rotate about the Y axis; first and second X-axis driving mechanisms for rotating the first and second rotating shafts about the X axis independently of each other; and a radome placed on the fixed base for covering the entire apparatus.
The antenna apparatus thus constructed allows each of the first and second antennas to rotate about each of the three axes independently, allowing the tracking of low-earth orbit satellites.
To attain the second object, there is provided a bent waveguide for transmitting two signals of different frequencies in the form of two polarized waves perpendicular to each other, characterized in that the waveguide is rectangular in cross section and its height and width are determined according to the polarized waves and the frequencies of the two signals.
The waveguide thus constructed allows the generation of the higher mode and crosstalk to be suppressed in its bends.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.